1. Field of the Invention
The invention relates generally to drill bits which have polycrystalline diamond compact cutters thereon.
2. Background Art
Polycrystalline diamond compact (PDC) cutters have been used in industrial applications including wellbore drilling and metal machining for many years. In these applications, a compact of polycrystalline diamond (or other superhard material such as cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide, to form a cutting structure. A compact is a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass.
An example of a use of PDC cutters is in a drill bit for earth formation drilling is disclosed in U.S. Pat. No. 5,186,268. FIG. 1 in the '268 patent shows a cross-section of a rotary drill bit having a bit body 10. A lower face of the bit body 10 is formed to include a plurality of blades (blade 12 is shown in FIG. 1) that extend generally outwardly away from a rotational axis 15 of the drill bit. A plurality of PDC cutters 20 are disposed side by side along the length of each blade. The number of PDC cutters 20 carried by each blade may vary. The PDC cutters 20 are affixed to a stud-like carrier, which may also be formed from tungsten carbide, and is received and secured within a corresponding socket in the respective blade.
A typical cutter 20 is shown in FIG. 2. The typically cutter 20 has a cylindrical cemented carbide substrate body 22 having an end face or upper surface 23 referred to herein as the “interface surface” 23. An ultra hard material layer (cutting layer) 24, such as polycrystalline diamond or polycrystalline cubic boron nitride layer, forms the working surface 25 and the cutting edge 26. A bottom surface 27 of the cutting layer 24 is bonded on to the upper surface 23 of the substrate 22. The joining surfaces 23 and 27 are herein referred to as the interface 28. The top exposed surface or working surface 25 of the cutting layer 24 is opposite the bottom surface 27. The cutting layer 24 typically may have a flat or planar working surface 25, or a non-planar surface (not shown separately).
Drill bits using conventional PDC cutters are sometimes unable to sustain a sufficiently low wear rate at the cutter temperatures generally encountered while drilling in abrasive and hard rock. These temperatures may affect the bit life, especially when the temperatures reach 700-750° C., resulting in structural failure of the PDC cutting layer. A PDC cutting layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are typically found within the interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table.
In order to obviate this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a hot strong acid, e.g., nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC layer. By leaching out the cobalt, a thermally stable polycrystalline (TSP) diamond layer is formed.
Alternatively, TSP may be formed by forming the diamond layer in a press using a binder other than cobalt, one such as silicon, which has a coefficient of thermal expansion more similar to that of diamond than cobalt has. During the manufacturing process, a large portion, 80 to 100 volume percent, of the silicon reacts with the diamond lattice to form silicon carbide which also has a thermal expansion similar to diamond. Upon heating, any remaining silicon, silicon carbide, and the diamond lattice will expand at more similar rates as compared to rates of expansion for cobalt and diamond, resulting in a more thermally stable layer. PDC cutters having a TSP cutting layer have relatively low wear rates, even as cutter temperatures reach 1200° C.
Thus, the methods for securing TSP to a rigid substrate for use in drill bit cutters have been actively investigated. In the attachment of PDC to a substrate, cobalt typically plays a significant role to bond the diamond to the substrate. However, because TSP is made by removing cobalt from the diamond layer, attachment of TSP to the substrate is significantly more complicated, as compared to the attachment of PDC to a substrate.
Brazing a TSP disc to a rigid substrate, having a relatively high modulus of elasticity, such as cobalt bonded tungsten carbide or molybdenum, may improve the performance of the TSP cutting elements, as compared to the performance of TSP cutting elements not having a rigid substrate. In the brazing process, a braze filler interlayer is positioned between the diamond layer and the substrate. The interlayer is melted and, upon subsequent solidification, is bonded to the diamond component and the substrate forming a braze joint. One braze filler metal composition that has been used to secure TSP to a substrate is a TiCuAg braze alloy. When using this composition, all components are heated slowly to 800-900° C. and melted to form discontinuous two-phase micro structures. Higher braze temperatures (such as over 1200° C.) cannot be used without resulting in TSP damage. Average shear strengths of the braze layer ranging from 20,000 to 35,000 psi have been achieved using direct resistance, induction and furnace heating methods. Most commercially available braze alloys result in a maximum shear strength of 35,000 psi.
The differential in the coefficients of thermal expansion between the substrate and the diamond layer often results in thermal residual stress. To minimize problems caused by thermal residual stress, a metal interlayer is included between two braze foils to control these stresses. See Ref. Brazing Handbook, American Welding Society, Ch. 30 Carbide Tools, 406 (1991). The thickness of the metal interlayer is typically about 50% of the entire joint thickness: for example, a 0.004 inches metal layer sandwiched between two 0.002 braze foils. U.S. Pat. No. 5,049,164 includes metallic bonding layers between tungsten and copper layers, which serve as a coating for bonding polycrystalline diamond to a matrix. The metallic bonding interlayers are taught to preferably be between 1.0 and 3.0 microns thick.
Cutter failure also results with fractures in the diamond layer caused by mechanical affects. Two of the mechanical affects which may lead to such fracture include vibration and impact, which may be termed “chatter.” Chatter can be defined as vibration with an amplitude that exceeds the depth of the cut. It may cause cutter damage via microfracture and reduce the rate of penetration of drilling. The vibrating chatter can cause a wavy surface on the bottom hole profile. When the wave of the leading cutter matches a trailing cutter, the cutting thickness is constant and results in a smoother cutting action. If the waves are out of sync, the chip thickness varies, and the regenerative vibration causes chatter. Chatter may be controlled by maintaining a constant chip thickness and by absorbing or redirecting the energy that generates chatter.
While these prior art methods provide satisfactory attachment of PDC or TSP to a substrate, there still exists a need for methods for attachment of TSP to a substrate with a stronger joint and an improved shear strength with reduced cracking in the cutting elements.